Level of significance without mean given

Hi, i can't remember how to do this problem and i have been having trouble finding an answer.

The question is "An SRS of size 15 is taken from a normal distribution with standard deviation of 2. It is decided to test H0:u=20 versus H1=u<20. Whats the critical value at a 5% level of significance"

Also "the value of the test statistic for the sample is -1.62 then the p value for the test is ..."

I know the answers are -1.645 and 0.0526 respectivley. Can anyone help me out?